Electromagnetic flowmeters for measuring the volumetric flow rates of fluids are used extensively in the process control industry. In a magnetic flowmeter, an magnetic field is generated having lines of flux which are perpendicular to the longitudinal axis of the flowtube through which the fluid to be metered is conducted. The flux lines are also perpendicular to the transverse axis along which a pair of measuring electrodes are located at diametrically-opposed positions with respect to the flowtube. An electromotive force induced in the fluid by the magnetic field and the moving fluid produces a voltage across the electrodes. That voltage is a function of the process flow rate.
In one type of magnetic flowmeter, the electrodes are electrically insulated from the fluid to be metered, by a dielectric material. Such an "non wetted" electrode configuration is advantageous in handling troublesome process fluids such as corrosive fluids, sewage and slurries which could damage and corrode the electrodes if in contact therewith.
In flowmeters having non wetted electrodes, the electrodes are either embedded in the flowtube or disposed on the exterior surface thereof. Either electrode configuration has problems associated therewith. For example, U.S. Pat. No. 4,658,652, Picone et al., discloses a flowmeter having a spool-shaped flowtube formed of a ceramic material and having a pair of electrodes disposed on the exterior surface thereof. The thickness of the ceramic material results in a weak signal at the electrodes, requiring greater amplification and processing of the signal. A reduction in the amount of material separating the electrodes from the process fluids increases the signal strength but decreases the structural integrity of the flowtube and its ability to withstand pressure exerted thereon by the process fluid.
Flowmeters in which the electrodes are embedded within the wall of the flowtube present unique manufacturing problems. For example, Japanese Laid-Open Patent Application No. 58-196419 discloses a method of manufacturing flowtubes characterized by forming an electrode, by a metalizing process, on the outer surface of a first tube of unfired ceramics, and fitting a second tube, also made of unfired ceramics, about the surface of the first tube. Both tubes are then fired together to form a whole unit. A disadvantage of this method is that voids or air pockets form intermediate the two tubes during the firing process. Such voids seriously jeopardize the integrity of the flowtube and its ability to withstand the pressure imparted thereto by the process. More importantly the voids may provide a path for process fluid to penetrate which renders the meter inoperable when the fluid reaches the electrodes.
International Patent Application WO87/01197 discloses a method for manufacturing a ceramic measurement tube in which powdered green ceramic material is placed in a mold with a pair of platinum wire grids located at specific positions. The unfired ceramic material is then compressed and fired. Such a method has several disadvantages. First, the precise placement of the platinum wire grids in a mold of raw ceramic material is very difficult. If the grids or electrodes are not diametrically opposed as precisely as possible, inaccurate flowtube measurements will result. Second, the use of a wire grid results in a relatively small electrode area and a correspondingly weaker signal. Third, platinum is a relatively expensive material and will substantially increase the production cost of the tube.
Accordingly, it is desirable to manufacture a flowtube for use with a magnetic flowmeter which is truly monolithic, with no seams exposed to the process fluid, and in which the electrodes are disposed in close proximity to, but not in contact with, the process fluid flowing therethrough.
A second problem associated with flowmeters having non wetted electrodes involves zero offsets present in the output signal of the flowmeter. Such offsets appear as voltages across the electrodes, despite the absence of any fluid flow through the flowmeter which, in an ideal flowmeter, should cause an output flow signal of zero volts. Zero offsets add an error component into the output flow signal from the flowmeter and often require external compensation circuitry to correct the output flow signal.
Accordingly, it is desirable to reduce the amount of zero offset in the output flow signal of a flowmeter.
It is, therefore, an object of the present invention to provide an electromagnetic flowmeter which can accurately measure the flow rate of a fluid passing therethrough.
Another object of the present invention is to provide an electromagnetic flowmeter which can accurately measure the flow rate of a fluid passing therethrough at temperatures up to 400.degree. F.
A further object of the present invention is to provide a monolithic flowtube for use in an electromagnetic flowmeter for measuring fluid flow rates.
Yet another object of the present invention is to provide a monolithic flowtube for use with an electromagnetic flowmeter which has a pair of diametrically opposed capacitive electrodes embedded therein.
Still a further object of the present invention is to provide a monolithic flowtube for use with an electromagnetic flowmeter having a pair of electrodes embedded therein, the electrodes comprising a relatively inexpensive conductive material.
Yet a further object of the present invention to provide a flowtube for use with an electromagnetic flowmeter in which the component parts of the flowtube are sintered together to form a monolithic flowtube body.